In recent years, as information-driven society moves forward, it is desired that an information recording method and an information storage device having remarkably high recording density are developed. Magnetic disk devices which access to information using a magnetic field are information storage devices capable of rewriting information. Among them, a floating head type magnetic disk device in which a head for applying a magnetic field is floated by air flow generated by rotation of a disk is widely used as a compact and high recording density information storage device.
In the floating head type magnetic disk device, if a head comes into contact with a rotating disk, information recorded in the disk is damaged. On the other hand, in order to efficiently apply magnetic field to a disk to enhance the accessing accuracy, it is necessary to bring the head closer to the disk, and a floating amount of a head is reduced to the order of some nm in recent years. Thus, although it is required to float a magnetic head from a disk precisely by a predetermined amount, but in the reality, the floating amounts of magnetic heads are varied respectively due to variations during production.
To solve the problem, a heater is incorporated in a magnetic head, and the magnetic head is thermally expanded by heat of the heater to adjust the floating amount. By supplying power suitable for the magnetic head to the heater, it is possible to adjust the floating amount of each magnetic head to a target floating amount, and to reduce the variations of accessing precision between the devices. As a method for calculating supply power to the heater, Japanese Patent Laid-open Application No. 2006-190454 describes a method for calculating a floating amount of a head using Wallace equation and calculating heater power when the target floating amount is realized. First, information recorded in a magnetic disk is read while varying power to be supplied to a heater, and an AGC gain value when the read signal is amplified by a gain variable amplifier such that the read signal becomes an analog signal of predetermined amplitude is obtained. Next, the obtained AGC gain value is substituted into the Wallace equation shown below, and the floating amount of the head is calculated.Δd=(λ/2π)×Ln(T1/T2)  (1)
Here, Δd represents a change of a floating amount of a head, λ represents a recording wavelength, T1 represents a previous AGC gain value, and T2 represent a current AGC gain value.
By repeating changing operation of supply power to the heater, reading of information in the magnetic head, and obtaining of the AGC gain value, a first graph showing a relation between the heater supply power and the AGC gain value is prepared, the AGC gain value in the first graph is substituted into the equation (1), the floating amount of the head is calculated, and a second graph showing a relation between the heater power and the head floating amount is prepared. A heater power value with which the target floating amount can be obtained on the second graph is obtained, and power supplied to the heater is adjusted to the obtained heater power value. With this, the head floating amount can be set to the target floating amount.
According to the method described above, however, since it is necessary to measure waveforms of input/output signals of the gain variable amplifier using an oscilloscope or the like and to obtain the AGC gain value every time when the supply power to the heater is changed, there is a problem that it takes time. Further, it is necessary to convert the obtained AGC gain value into the floating amount of the head every time, and it takes processing time. Since the equation (1) for converting the AGC gain value to the floating amount of the head is for logarithm calculation, there is a problem that the amount of calculation is extremely high and processing load is large.